PURPOSE: To contrive complete combustion as well as stabilized combustion by burning charged materials to be treated slowly to reduce affection of load fluctuation of the materials to be treated and contrive reduction of amount of pouring water for cooling the inside of a furnace by a method wherein waste gas recirculating gas is introduced into only the air box of a material to be treated charging zone of a fluidized bed furnace to fluidize fluidizing medium.

CONSTITUTION: The waste gas of combustion, which is cooled after recovering heat, is circulated and supplied to the small air box 24 of the lower side of a fluidized bed 12 in the materials to be treated charging zone of a fluidized bed furnace 10 while air is supplied into the other part of the air box to fluidize fluidizing medium. In this case, the temperature of the fluidized bed in the materials to be treated charging zone is controlled by regulating the supplying amount of combustion waste gas.